Alzheimer's disease (AD) affects greater than 12 million aging people worldwide. AD accounts for the majority of dementia clinically diagnosed after the age of 60. AD is generally characterized by the progressive decline of memory, reasoning, judgment and orientation. As the disease progresses, motor, sensory, and vocal abilities are affected until there is global impairment of multiple cognitive functions. The loss of cognitive function occurs gradually, typically leading to a diminished cognition of one's self, family and friends. Patients with severe cognitive impairment and/or diagnosed as end-stage AD are generally bedridden, incontinent, and dependent on custodial care. After initial diagnosis, the average survival period for the AD patient is only about nine to ten years. Due to the incapacitating, generally humiliating and ultimately fatal effects of AD, there is a need to effectively treat AD upon diagnosis.
AD is characterized by two major physiological changes in the brain. The first change, beta amyloid plaque formation, supports the “amyloid cascade hypothesis” which alleges that AD is caused by the formation of characteristic beta amyloid deposits (commonly referred to as beta amyloid “plaques” or “plaque deposits”) in the brain and in cerebral blood vessels (beta amyloid angiopathy). The second change in AD is the formation of intraneuronal tangles, consisting of an aggregate form of the protein tau. Amyloid plaques are thought to be specific for AD, while intraneuronal tangles are also found in other dementia-inducing disorders. Joachim et al., Alz. Dis. Assoc. Dis., 6:7-34 (1992).
It's been hypothesized that beta amyloid protein (also commonly referred to and referred to herein as amyloid-beta peptide or A-beta peptide) formation is a causative precursor or factor in plaque formation and consequently, the development and/or progression of AD. Deposition of A-beta peptide in areas of the brain responsible for areas of cognition, is believed to be a major factor in the development and/or progression of AD. Several lines of evidence indicate that progressive cerebral deposition of beta-amyloid peptide (A-beta) plays a seminal role in the pathogenesis of AD and can precede cognitive symptoms by years or even decades. Selkoe, Neuron, 6:487 (1991). Release of A-beta from neuronal cells grown in culture and the presence of A-beta in cerebrospinal fluid (CSF) of both normal individuals and AD patients has been demonstrated. Seubert et al., Nature, 359:325-327 (1992). In addition, autopsies of AD patients have revealed large numbers of lesions comprising these 2 factors in areas of the human brain believed to be important for memory and cognition. Smaller numbers of these lesions in a more restricted anatomical distribution are found in the brains of most aged humans who do not have clinical symptoms of AD. Amyloid containing plaques and vascular amyloid angiopathy were also found in the brains of individuals with Down's Syndrome, Hereditary Cerebral Hemorrhage with Amyloidosis of the Dutch-type (HCHWA-D), and other neurodegenerative disorders.
Beta amyloid plaques are primarily composed of amyloid beta peptide (A-beta peptide). A-beta peptide is derived from the proteolytic cleavage of a large transmembrane amyloid precursor protein (APP), and is a peptide ranging in about 39-42 amino acids. A-beta 42 (42 amino acids long) is thought to be the major component of these plaque deposits. Citron, Trends in Pharmacological Sciences, 25(2):92-97 (2004). Several aspartyl proteases are thought to be involved in the processing or cleavage of APP, resulting in the formation of A-beta peptide. Beta secretase (BACE, also commonly referred to as memapsin) is thought to first cleave APP to generate two fragments of the A-beta peptide: (1) a first N-terminus fragment and (2) a second C-99 fragment, which is subsequently cleaved by gamma secretase to generate the C-terminus fragment of the A-beta peptide. APP has also found to be cleaved by alpha-secretase to produce alpha-sAPP, a secreted form of APP that does not result in beta-amyloid plaque formation. This alternate pathway precludes the formation of A-beta peptide. A description of the proteolytic processing fragments of APP is found, for example, in U.S. Pat. Nos. 5,441,870, 5,712,130 and 5,942,400.
BACE is an aspartyl protease enzyme comprising 501 amino acids and responsible for processing APP at the beta-secretase specific cleavage site. BACE is present in two forms, BACE 1 and BACE 2, designated as such depending upon the specific cleavage site of APP. Beta secretase is described in Sinha et al., Nature, 402:537-554 (1999) (p 510) and PCT application WO 2000/17369. It has been proposed that A-beta peptide accumulates as a result of APP processing by BACE. Moreover, in vivo processing of APP at the beta secretase cleavage site is thought to be a rate-limiting step in A-beta production. Sabbagh, M. et al., Alz. Dis. Rev. 3:1-19 (1997). Thus, inhibition of the BACE enzyme activity is thought to be desirable for the treatment of AD.
Studies have shown that the inhibition of BACE may be linked to the treatment of AD. BACE1 knockout mice have failed to produce A-beta, and present a normal phenotype. When crossed with transgenic mice that over express APP, the progeny show reduced amounts of A-beta in brain extracts as compares with control animals (Luo et al., Nature Neuroscience, 4:231-232 (2001)). This evidence further supports the concept that inhibition of beta secretase activity and a corresponding reduction of A-beta in the brain should provide a therapeutic method for treating AD and other beta amyloid or plaque related disorders.
Researchers have taken several approaches to treat AD and plaque-related disorders. One approach has been to reduce the formation of plaque on the brain. Particularly, a common approach has been to inhibit the activity of beta secretase. For example, each of the following PCT publications: WO 03/045913, WO 04/043916, WO 03/002122, WO 03/006021, WO 03/002518, WO 04/024081, WO 03/040096, WO 04/050619, WO 04/080376, WO 04/099376, WO 05/004802, WO 04/080459, WO 04/062625, WO 04/042910, WO 05/004803, WO 05/005374, WO 03/106405, WO 03/062209, WO 03/030886, WO 02/002505, WO 01/070671, WO 03/057721, WO 03/006013, WO 03/037325, WO 04/094384, WO 04/094413, WO 03/006423, WO 03/050073, WO 03/029169 and WO 04/000821, describe inhibitors of beta secretase, useful for treating AD and other beta-secretase mediated disorders.